Polyurethane compositions that have isocyanate groups have been used in a wide variety of applications, for example, as adhesive, sealant, filling compound, coating or floor covering. While one-component polyurethane compositions can be used as such and cure after their application by contact with moisture and/or by means of heat, two-component polyurethane compositions include two components that are stored separately from one another—in general one component with free isocyanate groups and one component with free hydroxyl groups—which are thoroughly mixed for use of the composition just shortly before or during their application and thereupon cure. For two-component polyurethane compositions, it can be desirable that the compositions cure as quickly as possible in a sufficiently long pot life and processing time, for example, to build up strength as quickly as possible after application and thus to be available to be subject to load or working after a short time, thus can be moved with parts joined by adhesive or can be removed by attaching holding devices, or thus an applied coating can be walked on, recoated, or polished.
In a two-component polyurethane composition, the curing can be accelerated, for example, by supplying heat or by using strong or high-dose catalysts. The supply of heat can bring about an elevated cost of administration, can damage the substrates, and can result in the formation of bubbles and loss of adhesion, and the use of strong or high-dose catalysts can cause losses in the shelf life and the long-term stability of the composition, and can result, for example, in a shortening of the open time of the composition.
Two-component polyurethane compositions, whose hydroxyl-group-containing components in addition have free primary or secondary amino groups of jointly contained polyamines, can cure especially quickly, in this case have very little tendency to form bubbles and exhibit very good mechanical properties, since more urea groups are produced, which generally increase the strength and elasticity of polyurethanes. The reactivity of free amino groups relative to isocyanate groups can be high, however, in such a way that such compositions can have too short a pot life and processing time (open time) for most applications. In addition, amines in the air often tend to form crusts by reaction with CO2 (“blushing”) and/or can have disadvantages because of their toxicity relative to the operational safety and ecology.
Instead of free amines, so-called blocked amines with hydrolytically activatable amino groups can be used, for example in the form of imines or oxazolidines. The latter do not have any blushing and make possible two-component compositions with somewhat longer open times. The use of blocked amines in polyurethane compositions can result in other difficulties, for example, caused by the fact that the substances used for blocking the amino groups—typically aldehydes or ketones—can be released during the curing of the composition. In most cases, the latter are volatile and strong-smelling and can result in, for example, strong odors and irritation.
Two-component polyurethane compositions that contain aldimines with additional reactive groups, which cure quickly and odor-free, are known from WO 2007/036571 A1. During curing, these aldimines release a relatively low-volatile aldehyde, which remains in the cured composition and exerts a more or less pronounced softening action on the latter. As a result, these compositions remain limited with respect to their mechanical properties, for example, the achievable values for the modulus of elasticity, and therefore do not remain accessible to all applications.